1. Field of the Invention
The present invention relates to an isolation amplifier circuit, and particularly, to an isolation amplifier circuit for a noise cancel circuit of an audio apparatus such as a stereo sound system.
2. Description of the Prior Art
In a car audio apparatus such as a stereo sound system installed on a car, noise induced by an engine or other devices easily intrudes into an audio power amplifier since a car body is used as a ground portion of a circuit. As a result, sound quality is deteriorated. Thus, it is required to cancel the noise on the way to the audio power amplifier.
FIG. 1 shows a conventional car audio circuit having a noise cancel circuit. Numeral 1 is a tuner deck having amplifiers 4 and 5 respectively connected to input terminals 2 and 3 to which audio signals of left (L) and right (R) channels of a stereo are input. An output thereof is connected to input terminals 9 and 10 of a noise cancel circuit 8 through capacitors 6 and 7. The noise cancel circuit 8 is formed with an integrated circuit (IC). The input terminals 9 and 10 are connected to plus terminals of buffers 11 and 12. Output terminals of buffers 11 and 12 are connected to plus terminals of operation amplifiers 13 and 14 through resistors R3 and R3'. Minus input terminals of the operation amplifiers 13 and 14 are connected to output terminals through resistors R2 and R2'. They are connected also to a first common ground terminal 15 through resistors R1 and R1'. To the first common ground terminal 15, a capacitor 16 is connected. The other end of the capacitor 16 is connected, together with the tuner block 1, to a car body 18 at ground point 17. Plus input terminals of the operation amplifiers 13 and 14 are connected to a second common ground terminal 19 through resistors R4 and R4'. To the second common ground terminal 19, a capacitor 20 is connected. The output terminals of the operation amplifiers 13 and 14 are connected to output terminals 21 and 22. The output terminals 21 and 22 are connected to audio power amplifiers 25 and 26 through capacitors 23 and 24. To the power amplifiers 25 and 26, speakers 27 and 28 are connected as shown in the figure.
Next, operation of the noise cancel circuit 8 in the car audio circuit will be described. Noise from a noise source 29 is equivalent to noise which exists in the car body 1. The noise is divided into several elements: a) noise which intrudes from the ground point 17 into the plus input terminal of the operation amplifier 13 for the L channel by way of the tuner deck 1, the amplifier 4, the terminal 9, and the buffer 11; b) noise which intrudes from the ground point 17 into the plus input terminal of the operation amplifier 14 for the R channel by way of the tuner deck 1, the amplifier 5, the terminal 10, and the buffer 12; c) noise which intrudes from the ground point 17 into the minus input terminal of the operation amplifier 13 for the L channel by way of the capacitor 16, the first common terminal 15, and the resistor R1; and d) noise which intrudes from the ground point 17 into the minus input terminal of the operation amplifier 14 for the R channel by way of the capacitor 16, the first common terminal 15, and the resistor R1'.
Only cancellation of noise of the L channel will be described, since the principle of noise cancellation of the R channel is the same as that of noise cancellation of the L channel. As described above, noise intrudes into the plus and minus input terminals of the operation amplifier 13 for the L channel. If the noise intruding into the plus input terminal and the noise intruding into the minus input terminal are equal to each other in phase and size, the noises are canceled by a common-mode input rejection action of the operation amplifier 13 for the L channel, so that the noises are not included in an output of the operation amplifier 13 for the L channel.
For the above-described noise cancellation, it is required that the equation R1.times.R4=R3.times.R2 (i.e. R1/R2=R3/R4) is fulfilled with respect to the resistors, and that e1=e2 where e1 and e2 are noise signal voltages of the plus and minus input terminals.
For the above-described conventional art, the capacity of the capacitor 16 connected to the first common ground terminal 15 has to be large. This is because if the capacity of the capacitor 16 is small, impedance is large so that R1/R2=R3/R4 which is one of the conditions for noise cancellation cannot be fulfilled since the capacitor 16 is equivalently included by impedance of the resistor R1. For this reason, the capacity of the capacitor 16 has to be large (impedance has to be small).
Moreover, in a circuit, for example, where the capacitor 16 is not connected to the first common ground terminal 15, noise cancel is interfered with when impedance connected to the first common ground terminal 15 is large.